The present invention relates to a multi-valve engine having three valves consisting of either intake valves or exhaust valves.
As disclosed in Japanese Patent Application Laying-open Gazette No. 62-78453, a conventional multi-valve engine is generally provided with a set of three intake valves and two exhaust valves on each cylinder for the purpose of obtaining higher intake and exhaust efficiency.
In this type of engine, the three intake valves are oriented such that valve stems thereof are inclined outwardly to one side of a cylinder bore and are parallel to each other, while the two exhaust valves are oriented such that valve stems thereof are inclined outwardly to an opposite side of the cylinder bore. Lower surfaces of valve heads of the intake and exhaust valves are disposed into an angular shape so as to form a pent-roof type efficient combustion chamber.
The intake valves are driven by an intake camshaft through rocker arms and the exhaust valves are actuated by an exhaust camshaft through rocker arms. In detail, intake and exhaust rocker arm shafts are arranged in the cylinder line direction in farther positions from a center of the cylinder bore than the intake and exhaust valves. One end of each rocker arm is swingably supported by the rocker arm shaft, and the other end thereof is disposed on an end portion of a front shaft of the intake and the exhaust valves. A cam of the camshaft disposed on an upper side of the rocker arm makes contact with the central part of the rocker arm. Accordingly, the rocker arm swings along with the rotation of the camshaft, thus initiating vertical motion of the intake and exhaust valves.
In order to actuate the intake and the exhaust valves under a condition free from a valve lash, the inventors came up with the idea of using a different type of rocker arm from the conventional rocker arm in the above-mentioned multi-valve engine. They provided a rocker arm in such a way that one end thereof is supported on a supporting portion of hydraulic lash adjuster means and the other end thereof is mounted respectively on the end portions of the valve stems of the intake and the exhaust valves. The mechanism adopted is that the cam of the camshaft on the upper side of the rocker arm makes contact, from the upper direction, with substantially the center of the rocker arm to swing the rocker arm on the supporting portion of the hydraulic lash adjuster means as its axis along with the rotation of the camshaft, and thereby initiating the vertical motion of the intake and the exhaust valves.
However, the disadvantage of this mechanism is that since the three intake valves are arranged on one side of the cylinder bore at each cylinder, the size of the engine becomes substantially larger if any modification in the layout of the rocker arm and the hydraulic lash adjuster means is not undertaken. If the engine becomes larger, the original object of improving the intake and exhaust efficiency and combustion efficiency cannot be achieved.
Generally, the hydraulic lash adjuster means comprises a cylindrical casing having a bottom thereof, an inner sleeve attached slidably inside the casing. In this hydraulic lash adjuster means, a supporting portion including a pivot for supporting the rocker arm is further provided on a head portion of the inner sleeve. Then, a high hydraulic pressure is introduced to a hydraulic chamber formed between the casing and the inner sleeve to push the inner sleeve out of the casing. Thus, a force acting between the rocker arm and the supporting portion, namely, the supporting force, can be kept at a higher level. Consequently, a clearance between the rocker arm and the camshaft, and a clearance between the rocker arm and the intake and the exhaust valve become zero.
In the above-structured hydraulic lash adjuster means, the inner sleeve repeats a reciprocating motion in the casing. Accordingly, when given a force intersecting with the reciprocating motion thereof, i.e. a lateral force, by the supporting force, the inner sleeve vibrates due to its pivotal swing movement during the reciprocating motion thereof. As a result, a sliding contact surface of the inner sleeve is subject to an excessive wear, causing troubles like an oil leak from the hydraulic chamber. This will result in lower durability and reliability of the hydraulic lash adjuster means.